Current production and drilling columns are generally linked to the seal bottom by a link composed of a pot type joint or flexible joint which allows for an angular movement of about 10.degree. in any direction. The exception to this rule is provided by the tension-leg platform (TLP) installed on the HUTTON bed in the North Sea where production columns are embedded directly in wet wellheads which are combined on the platform vertical line.
Such embedding offers advatnages for several reasons. Firstly, it avoids imposing any significant bending on production pipes inside the column. It also reduces the angular clearances of the column. Finally, it is more compact and less expensive and requires less maintenance than a pot type joint.
Where a drilling extension piece or drilling riser is used, such a link considerably reduces the wear of rods.
The drawback of embedding is that the moments, induced by the lateral yield of the platform and via the effecft of the sea current, amy be extremely considerable. In order to reduce the bending stresses, which otherwise would exceed the limit allowed in the column, it is essential to introduce a variable rigidity element between the column and the embedding.
The element may be designed so that the curve provoked is roughly constant along its entire length. This requires that the bending ridigity (EI) moves around precisely along the element.
The data of the problem are the follow:
______________________________________ A.sub.B = the root angle of the column in the case of a non-rigid joint, (EI).sub.R = the column bending rigidity, (EI).sub.O = upper extremity bending rigidity of variable rigidity element, M.sub.O = max. permissible moment on joint between column and element, T = traction force on joint and C.sub.O = shearing force on joint ______________________________________
and by posing ##EQU1## To best understand these designations, one should refer to FIG. 6 of this application.
It can be demonstrated that the following relations are roughly exact: ##EQU2## The length (L) required for the element ##EQU3## The angle (A.sub.e) for which the element must bend is ##EQU4## Required evolution of bending radius (EI) along element ##EQU5## Max. moment at lower extremity of element ##EQU6## From the equation (2), it is deducted that the smaller is the minimum bending ridigity (EI).sub.0, the shorter may be the element.
The equation (5) shows that the maximum moment transmitted to the foundation is in direct relation with the length (L) of the element and permitted bending radius (Re). It is thus an advantage to make this element as flexible as possible.
Where the column is made from a flexible material such as, for example, a composite material (carbon fibers/glass fibers/resin) and the element consists of a stiff material, such as steel for example, it is possible that the permissible moment (M.sub.O) at the joint between them is much smaller than in the element.
There are two possible solutions.
The element may be made longer than it needs to be. The alternative is to introduce a universal joint several metres long with a constant section between the column and the variable rigidity element.
The best solution in order to avoid having to introduce a universal joint between the column and variable rigidity element is to ensure that the upper extremity of the element is at least as bending flexible as the column itself. Where a column comprises composite materials (carbon fibers/glass fibers/resin), it is difficult to obtain the required flexibility. The ideal solution would be to make the element also of composite material.
Currently, it is difficult to produce such a part with the required rigidity variation.
In the case of a titanium or steel element, the required minimum thickness as regards its mechanical behaviour results in a bending rigidity much greater than that of a composite material column. This invention resolves the problem.